Variable scope

The global keyword

First, an example use of global:

Example 7-1. Using global <?php $a = 1; $b = 2; function Sum() { global $a, $b; $b = $a + $b; } Sum(); echo $b; ?>

The above script will output "3". By declaring $a and $b global within the function, all references to either variable will refer to the global version. There is no limit to the number of global variables that can be manipulated by a function.

A second way to access variables from the global scope is to use the special PHP-defined $GLOBALS array. The previous example can be rewritten as:

Example 7-2. Using $GLOBALS instead of global <?php $a = 1; $b = 2; function Sum() { $GLOBALS["b"] = $GLOBALS["a"] + $GLOBALS["b"]; } Sum(); echo $b; ?>

The $GLOBALS array is an associative array with the name of the global variable being the key and the contents of that variable being the value of the array element. Notice how $GLOBALS exists in any scope, this is because $GLOBALS is a superglobal. Here's an example demonstrating the power of superglobals:

Example 7-3. Example demonstrating superglobals and scope <?php function test_global() { // Most predefined variables aren't "super" and require // 'global' to be available to the functions local scope. global $HTTP_POST_VARS; print $HTTP_POST_VARS['name']; // Superglobals are available in any scope and do // not require 'global'. Superglobals are available // as of PHP 4.1.0 print $_POST['name']; } ?>

Using static variables

Another important feature of variable scoping is the static variable. A static variable exists only in a local function scope, but it does not lose its value when program execution leaves this scope. Consider the following example:

Example 7-4. Example demonstrating need for static variables <?php function Test () { $a = 0; echo $a; $a++; } ?>

This function is quite useless since every time it is called it sets $a to 0 and prints "0". The $a++ which increments the variable serves no purpose since as soon as the function exits the $a variable disappears. To make a useful counting function which will not lose track of the current count, the $a variable is declared static:

Example 7-5. Example use of static variables <?php function Test() { static $a = 0; echo $a; $a++; } ?>

Now, every time the Test() function is called it will print the value of $a and increment it.

Static variables also provide one way to deal with recursive functions. A recursive function is one which calls itself. Care must be taken when writing a recursive function because it is possible to make it recurse indefinitely. You must make sure you have an adequate way of terminating the recursion. The following simple function recursively counts to 10, using the static variable $count to know when to stop:

Example 7-6. Static variables with recursive functions <?php function Test() { static $count = 0; $count++; echo $count; if ($count < 10) { Test (); } $count--; } ?>

References with global and static variables

The Zend Engine 1, driving PHP4, implements the static and global modifier for variables in terms of references. For example, a true global variable imported inside a function scope with the global statement actually creates a reference to the global variable. This can lead to unexpected behaviour which the following example addresses:

<?php
function test_global_ref() {
    global $obj;
    $obj = &new stdclass;
}

function test_global_noref() {
    global $obj;
    $obj = new stdclass;
}

test_global_ref();
var_dump($obj);
test_global_noref();
var_dump($obj);
 ?>

Executing this example will result in the following output:

NULL
object(stdClass)(0) {
 }

A similar behaviour applies to the static statement. References are not stored statically:

<?php
function &get_instance_ref() {
    static $obj;

    echo "Static object: ";
    var_dump($obj);
    if (!isset($obj)) {
        // Assign a reference to the static variable
        $obj = &new stdclass;
    }
    $obj->property++;
    return $obj;
}

function &get_instance_noref() {
    static $obj;

    echo "Static object: ";
    var_dump($obj);
    if (!isset($obj)) {
        // Assign the object to the static variable
        $obj = new stdclass;
    }
    $obj->property++;
    return $obj;
}

$obj1 = get_instance_ref();
$still_obj1 = get_instance_ref();
echo "\n";
$obj2 = get_instance_noref();
$still_obj2 = get_instance_noref();
 ?>

Executing this example will result in the following output:

Static object: NULL
Static object: NULL

Static object: NULL
Static object: object(stdClass)(1) {
  ["property"]=>
  int(1)
 }

This example demonstrates that when assigning a reference to a static variable, it's not remembered when you call the &get_instance_ref() function a second time.